1. Field of the Invention
The present invention relates to a process for melting an electroconductive material in a cold crucible induction melting furnace and a melting furnace for carrying out the process.
2. Related Art
A process for melting an electroconductive material in a cold crucible induction melting furnace is known. The process comprises heating a mass of an electroconductive material to its melting temperature, decanting the solid inclusion particles contained in the liquid electroconductive material, and pouring a part of the mass of the liquid electroconductive material through a pouring conduit disposed under the melting furnace.
This process is usually employed for effecting a stabilized pouring of a molten metal with a variable pouring rate for producing metallic powders by atomization.
For this purpose, induction melting furnaces are known in which there is employed a crucible which receives an electroconductive material and is termed a cold crucible since it is permanently cooled.
In such furnaces, the partial or total melting of a mass of liquid electroconductive material is produced by an electromagnetic confinement so as to separate the mass of the liquid electroconductive material from the wall of the crucible.
To this end, the crucible comprises a plurality of metal sectors electrically insulated from one another and surrounded by means for the electromagnetic induction heating of the electroconductive material contained in the crucible.
The crucible is for example of cylindrical shape and comprises a substantially hemispherical or frustoconical bottom provided with a pouring orifice to which is fixed a tube for pouring the mass of liquid electroconductive material.
Cold metal crucible induction melting furnaces are preferred to refractory crucible furnaces which pollute the mass of liquid electroconductive material owing to contact between the material and the refractory walls of the crucible.
The pollution is due to the formation of inclusion particles of compounds which are for example oxidized compounds.
In particular applications, for example in the preparation of powders by the atomization of metals, this pollution incorporates into the powders many inclusions and it is in particular acknowledged that the presence of such inclusions in rotating parts of an aeronautic engine, for example based in nickel, may be the origin of defective behaviour in service of these parts subjected to fatigue stresses and in particular may result in premature fractures of the parts subjected to high stresses at high temperature.
In order to avoid these drawbacks, there have been proposed solutions - not fully satisfactory - based on the use of an electromagnetic nozzle forming the pouring orifice for the mass of liquid electroconductive material in which the latter has no contact with the walls of the nozzle.
In the field of the regulation of the rate of flow of a liquid metal through a pouring tube, patents FR-A-2 316 026, FR-A-2 396 612 and FR-A-2 397 251 also disclose electromagnetic devices operating at high frequency in which a copper screen must be used to obtain the desired confinement.
However, the adoption on an industrial scale of such a device, such as in an installation for the atomization of nickel-based superalloy powders, presents serious difficulties.
In order to avoid these difficulties, there is known from patent FR-A-2 649 625 an electromagnetic nozzle comprising a coiled inductor associated with a device for concentrating the magnetic field disposed between the coiled inductor and the walls of the pouring orifice.
Such a nozzle has the drawback of being conditioned, in its operation, by the choice of specific parameters of size, and parameters of definition of the magnetic field applied, such as for example the frequency and the intensity of said magnetic field.
Moreover, this nozzle has a large size and low efficiency.
Further, there is also known from patent FR-A-2 665 249 a cold crucible furnace associated with a magnetic yoke which makes it possible to bring the field lines closer together at the level of the upper surface of the molten charge contained in the crucible.
This bringing together of the field lines results in a centripetal driving of the molten matter at the level of the surface of the molten material, which results in a stirring of the molten charge in a direction opposed to the natural stirring direction produced in the absence of such a yoke.
This centripetal motion at the level of the upper surface of the molten charge enables the substances which are not yet perfectly melted and float on the surface of the charge, to be brought to the center and thereafter swallowed up in this charge and consequently permits a stirring of the mass of molten materials without taking into account inclusions present in the molten mass.
There is also known from patent FR-A-2 646 858 a process for decanting the inclusions of a molten metallic mass comprising employing a displacement of the inclusion particles toward the surface, in the thickness of the electromagnetic skin, and then a trapping of the particles by the coldest particles of the crucible.
In this process, two phenomena are employed, namely an electromagnetic stirring which brings the inclusions within the mass of molten metal toward the region of the electromagnetic skin and a capturing of the inclusions in the region of the skin, these inclusions being moved toward the wall of the crucible and the surface of the molten metal mass under the effect of the magnetic pressure forces.